Question

I want to retrieve state from a function object. But the function object has been casted to a function<> template. How can I do it?

I mean:

function object:

class Counter {
private:
    int counter_;
public:
    Counter():counter_(0) {cout << "Constructor() called" << endl;}
    Counter(int a):counter_(a) {cout << "Constructor(a) called" << endl;}

    void operator()(int sum) {
        cout << counter_++ + sum << endl;
    }

int getCounter() { return counter_;}
};

In main. My first step is use object directly:

int main() {
    Counter a(10);
    a(0);
    a(0);

    cout << "Counter: " << a.getCounter() << endl;

Its shows:

Constructor(a) called

10

11

Counter: 12

It’s ok and it’s what I expected.

But in

Counter b(10);
function<void(int)> f = b;
f(0);
f(0);
cout << "Counter: " << b.getCounter() << endl;

It shows

Constructor(a) called

10

11

Counter: 10

Argh!, I supposed that f was a wrapper of real object so modifying f we really modify b. No: f has a copy of b, but I can’t call to f.getCounter() so How can I get State (counter_ var) from f?

I can’t use directly Counter class (in this example) because I have some other similar classes with same signature “void(int)” And I want to use them in indistinctly in a caller function.

I can avoid std::function template at all using a common base class for all my function object but I think there is a solution more C++11 with STL and templates…

So, Is there that solution?

Thanks

Answer 1

This is not a cast:

function<void(int)> f = b;

A std::function is been constructed, with a copy of your function object as its target.

You can either make the target a reference to your function object:

function<void(int)> f = std::ref(b);

Or let f contain a copy but then retrieve that copy from f:

Counter* c = f.target<Counter>();
cout << "Counter: " << c->getCounter() << endl;

It’s usually safer to let f contain a copy as you don’t ned to worry about the lifetime of b.